Currently, a wide variety of fan drives are utilized for powering fans for cooling systems of internal combustion engines and the like, including direct engine drives, hydraulic drives, electric drives, and combinations of the above, both through direct connection to the fan and via a variety of clutch arrangements. Reference for instance Ishikawa et al. U.S. Pat. No. 3,853,098 issued Dec. 10, 1974; Sakasegawa et al. U.S. Pat. No. 3,894,521 issued Jul. 15, 1975; Kinder U.S. Pat. No. 4,223,646 issued Sep. 23, 1980; Clemente U.S. Pat. No. 4,461,246 issue Jul. 24, 1984; Merz U.S. Pat. No. 4,709,666 issued Dec. 1, 1987; and Suzuki et al. U.S. Pat. No. 4,941,437 issued Jul. 17, 1990.
In many instances it is desirable to be able to vary the rotational speed of the fan relative to the rotational speed of the engine or other power source therefor, for instance, to reduce fan speed when cooling demand is less. Fan noise is also reduced at lower fan speeds.
Slipping frictional clutches and the like are well known devices utilized for varying fan speed. However, for larger fan applications, such as for cooling large internal combustion engines such as the engines of off-highway trucks and the like, slipping frictional clutches have been found to wear out faster than desired due primarily to the power dissipation requirements to achieve lower fan speed. For instance, some known engine driven cooling fans for such large off-highway trucks operate at rotational speeds of as high as 600 revolutions per minute (rpm) during periods of maximum cooling need, which can require drive power of as much as 150 horsepower. For times when cooling demand is lower, but the rotational speed of the engine remains the same, as much as 50 horsepower may be required to be dissipated, which can rapidly consume frictional wear surfaces of a clutch. Additionally, the power dissipated is typically lost as heat.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.